1. Field of the Invention
This invention relates to a dental prosthetic composition containing a high proportion of a comprosite filler as an inorganic filler. More particularly, it relates to a photopolymerizable dental prosthetic composition which can readily be polymerized and cured by exposure to visible light to give a cured product having high mechanical properties and good water resistance.
2. Description of the Prior Art
It is important that dental prosthetic materials have not only good physical properties such as mechanical strength, abrasion resistance, water resistance and adhesion properties, but also excellent aesthetic properties such as transparency and polishability so as to bear greater resemblance to natural teeth. In order to provide satisfactory substitutes for amalgam, many attempts have been made to develop so-called composite resins formed by blending an inorganic filler with an organic resin in order to improve the aforesaid properties.
Such composite resins consist of three main components: (1) an inorganic filler, (2) an olefinically unsaturated compound and (3) a polymerization initiator.
However, when an inorganic filler is simply blended with an organic resin, the resulting composite resin has poor interfacial affinity (i.e., poor compatibility and adhesion properties between the inorganic filler and the organic resin) and fails to exhibit satisfactorily high performance. Thus, in order to overcome this shortcoming, there have been proposed dental materials containing a filler which have undergone a surface treatment, for example, with a silane coupling agent. However, the surface treatment with a silane coupling agent has the great disadvantage that its applicability is limited because it is only effective for glassy fillers having silanol groups in the surface. Moreover, since the silonxane bonds so formed are subject to hydrolysis, the resulting treated filler has poor water resistance. Accordingly, in the case of quartz-based fillers commonly used in dental prosthetic compositions chiefly for the purpose of imparting thereto high hardness and desirable aesthetic properties, it is difficult to achieve a desired interfacial reinforcing effect. Further, such treated fillers are also disadvantageous from a clinical point of view because, during long-term use in the wet oral cavity, their poor water resistance causes a reduction in mechanical strength with time.
In addition, much research has been made into the monomers constituting the aforesaid organic resin. For example, it has been found that bisphenol A-derived multifunctional monomers, as typified by bisphenol A diglycidyl dimethacrylate (hereinafter referred to as Bis-GMA), can provide composite resins exhibiting a low degree of shrinkage on polymerization, and that hydrophobic monomers such as 2,2-bis[4-methacryloxyethoxy)phenyl]propane (hereinafter referred to as Bis-MEPP) can provide composite resins having excellent water resistance. However, Bis-GMA is disadvantageous in that, since it has a hydroxyl group in the molecule, the resulting cured product exhibits high water absorption and hence poor water resistance. On the other hand, Bis-MEPP has low curability and fails to give a satisfactorily high cross-linking density, so that the resulting cured product has poor mechanical strength. Further, attempts have been made to overcome these disadvantages by the combined use of Bis-GMA and Bis-MEPP, but a mere combination thereof has failed to provide a composite resin having excellent properties. In the existing state of the art, therefore, a very wide range of monomer compositions must be examined to determine the monomer composition most suitable for the intended purpose.